Gaseous waste products resulting from the combustion of hydrocarbonaceous fuels, such as gasoline and fuel oils, comprise carbon monoxide, hydrocarbons and nitrogen oxides as products of combustion or incomplete combustion, and pose a serious health problem with respect to pollution of the atmosphere. While exhaust gases from other carbonaceous fuel-burning sources, such as stationary engines, industrial furnaces, etc., contribute substantially to air pollution, the exhaust gases from automotive engines are a principal source of pollution. Because of these health problem concerns, the Environmental Protection Agency (EPA) has promulgated strict controls on the amounts of carbon monoxide, hydrocarbons and nitrogen oxides which automobiles can emit. The implementation of these controls has resulted in the use of catalytic converters to reduce the amount of pollutants emitted from automobiles.
In order to achieve the simultaneous conversion of carbon monoxide, hydrocarbon and nitrogen oxide pollutants, it has become the practice to employ catalysts in conjunction with air-to-fuel ratio control means which functions in response to a feedback signal from an oxygen sensor in the engine exhaust system. Although these three component control catalysts work quite well after they have reached operating temperature of about 300.degree. C., at lower temperatures they are not able to convert substantial amounts of the pollutants. What this means is that when an engine and in particular an automobile engine is started up, the three component control catalyst is not able to convert the hydrocarbons and other pollutants to innocuous compounds. Despite this limitation, current state of the art catalysts are able to meet the current emission standards. However, California has recently set new hydrocarbon standards (these standards most probably will be promulgated nationwide) which can not be met with the current state of the art three component control catalysts.
Applicant has found a solution to this problem which involves the use of an adsorbent bed to adsorb the hydrocarbons during the cold start portion of the engine. Although the process will be exemplified using hydrocarbons, the instant invention can also be used to treat exhaust streams from alcohol fueled engines as will be shown in detail. Applicant's invention involves placing an adsorbent bed immediately before the catalyst. Thus, the exhaust stream is first flowed through the adsorbent bed and then through the catalyst. The adsorbent bed preferentially adsorbs hydrocarbons over water under the conditions present in the exhaust stream. After a certain amount of time, the adsorbent bed has reached a temperature (about 150.degree. C.) at which the bed is no longer able to remove hydrocarbons from the exhaust stream. That is, hydrocarbons are actually desorbed from the adsorbent bed instead of being adsorbed. This regenerates the adsorbent bed so that it can adsorb hydrocarbons during a subsequent cold start.
The adsorbents which may be used to adsorb the hydrocarbons may be selected from the group consisting of molecular sieves which have 1) a Si:Al ratio of at least 2.4; 2) are hydrothermally stable; and 3) have a hydrocarbon selectivity greater than 1. Examples of molecular sieves which meet these criteria are silicalite, faujasites, clinoptilolites, mordenites and chabazite. The adsorbent bed may be in any configuration with a preferred configuration being a honeycomb monolithic carrier having deposited thereon the desired molecular sieve.
The prior art reveals several references dealing with the use of adsorbent beds to minimize hydrocarbon emissions during a cold start engine operation. One such reference is U.S. Pat. No. 3,699,683 in which an adsorbent bed is placed after both a reducing catalyst and an oxidizing catalyst. The patentees disclose that when the exhaust gas stream is below 200.degree. C. the gas stream is flowed through the reducing catalyst then through the oxidizing catalyst and finally through the adsorbent bed, thereby adsorbing hydrocarbons on the adsorbent bed. When the temperature goes above 200.degree. C. the gas stream which is discharged from the oxidation catalyst is divided into a major and minor portion, the major portion being discharged directly into the atmosphere and the minor portion passing through the adsorbent bed whereby unburned hydrocarbon is desorbed and then flowing the resulting minor portion of this exhaust stream containing the desorbed unburned hydrocarbons into the engine where they are burned.
Another reference is U.S. Pat. No. 2,942,932 which teaches a process for oxidizing carbon monoxide and hydrocarbons which are contained in exhaust gas streams. The process disclosed in this patent consists of flowing an exhaust stream which is below 800.degree. F. into an adsorption zone which adsorbs the carbon monoxide and hydrocarbons and then passing the resultant stream from this adsorption zone into an oxidation zone. When the temperature of the exhaust gas stream reaches about 800.degree. F. the exhaust stream is no longer passed through the adsorption zone but is passed directly to the oxidation zone with the addition of excess air.
Finally, Canadian Patent No. 1,205,980 discloses a method of reducing exhaust emissions from an alcohol fueled automotive vehicle. This method consists of directing the cool engine startup exhaust gas through a bed of zeolite particles and then over an oxidation catalyst and then the gas is discharged to the atmosphere. As the exhaust gas stream warms up it is continuously passed over the adsorption bed and then over the oxidation bed.
The problem with the prior art processes is that the adsorbents which were used are not selective. That is, water is adsorbed as easily as the pollutants which necessitates the use of large beds, which in turn means that the large bed acts as a heat sink, thereby cooling the exhaust and lengthening the time required to warm up the catalyst bed. Another problem with the adsorbents used in the prior art is that they have poor thermal durability and would not be able to meet the EPA durability requirements of at least 50,000 miles or 5 years. For these reasons, adsorbent beds have not been used in conjunction with catalysts to treat automotive exhaust streams.
Applicants have recognized this longfelt need and have solved the problems found in the prior art. This has been accomplished by the use of molecular sieves which selectively adsorb hydrocarbons and other pollutants over water. What this means is that the molecular sieve bed does not have to be very large in order to adsorb sufficient quantities of hydrocarbons and other pollutants during engine startup. Accordingly, the size of the adsorbent bed can be minimized so that the catalyst bed warms up as quickly as possible. Additionally, the molecular sieves which are used as the adsorbent are hydrothermally stable at temperatures of at least 750.degree. C. This means that the adsorbent bed can meet the EPA requirements that catalysts have a durability of at least 50,000 miles or 5 years.